Oxygen tailoring of polyethylene resins

ABSTRACT

A process is provided for extruding a bimodal polyethylene resin. The process includes providing a polyethylene homopolymer or copolymer resin having a bimodal molecular weight distribution; conveying the resin through an extruder having a feed zone in which the resin is not melted, a melt-mixing zone in which at least a portion of the resin is melted, and a melt zone in which the resin is in a molten state, each zone being partially filled with the resin; and contacting the molten resin in the melt zone with a gas mixture of 8 to 40% by volume O 2 . The resin can be further pelletized. The oxygen-tailored resin can be used to make polyethylene films having improved bubble stability.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of International ApplicationNo. PCT/US02/32243, filed Oct. 9, 2002, hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention is directed to methods of extruding polyethylenehomopolymer and copolymer resins. More particularly, the inventionprovides methods of oxygen tailoring polyethylene resins to improve thebubble stability and gauge uniformity of films made from such resins.

BACKGROUND

Tailoring of resins, such as polyethylene homopolymer or copolymerresins, is a well-known method of altering the molecular architectureand thus the bulk properties of the resin and of films and articles madetherefrom. Tailoring involves treating the resin with an agent, such asa peroxide or oxygen, capable of controlled degradation of the resin.The effect of tailoring on the resin rheological properties can be seenin an increase in shear thinning behavior, an increase in elasticity, anincrease in melt tension, a reduction in swelling during blow molding,and an increase in bubble stability during film blowing. Although notwishing to be bound by theory, it is believed that an effect oftailoring is to introduce low levels of long chain branching in theresin.

Polyolefin resins having bimodal molecular weight distributions and/orbimodal composition distributions are desirable in a number ofapplications. Resins including a mixture of a relatively highermolecular weight polyolefin and a relatively lower molecular weightpolyolefin can be produced to take advantage of the increased strengthproperties of higher molecular weight resins and articles and films madetherefrom, and the better processing characteristics of lower molecularweight resins.

Bimodal resins can be produced in tandem reactors, such as tandem gasphase reactors or tandem slurry reactors. Alternatively, bimetalliccatalysts such as those disclosed in U.S. Pat. Nos. 5,032,562 and5,525,678, and European Patent EP 0 729 387, can produce bimodalpolyolefin resins in a single reactor. These catalysts typically includea non-metallocene catalyst component and a metallocene catalystcomponent which produce polyolefins having different average molecularweights. U.S. Pat. No. 5,525,678, for example, discloses a bimetalliccatalyst in one embodiment including a titanium non-metallocenecomponent which produces a higher molecular weight resin, and azirconium metallocene component which produces a lower molecular weightresin. Controlling the relative amounts of each catalyst in a reactor,or the relative reactivities of the different catalysts, allows controlof the bimodal product resin.

A particularly useful application for bimodal polyethylene resins is infilms. Frequently, however, the bubble stability and gauge uniformity ofmedium density polyethylene (MDPE) resins and high density polyethylene(HDPE) resins are not adequate for producing thin films. Attempts havebeen made to tailor polyethylene resins to improve bubble stability,gauge uniformity, and/or otherwise improve resin or film properties;see, e.g., European Patent Publication No. EP 0 457 441, and U.S. Pat.Nos. 5,728,335; 5,739,266; and 6,147,167. Other background referencesinclude FR 2,251,576; EP 0 180 444; U.S. Pat. No. 5,578,682; EP 0 728796; and GB 1,201,060. However, it would be desirable to have improvedmethods of extruding polyethylene, particularly pelletized bimodalpolyethylene film resin, to provide resins having improved bubblestability and gauge uniformity when processed into film.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a process for extruding abimodal polyethylene resin. The process includes providing apolyethylene homopolymer or copolymer resin having a bimodal molecularweight distribution; conveying the resin through an extruder having afeed zone in which the resin is not melted, a melt-mixing zone in whichat least a portion of the resin is melted, and a melt zone in which theresin is in a molten state, each zone being partially filled with theresin; and contacting the molten resin in the melt zone with a gasmixture of 8 to 40% by volume O₂. The resin can further be pelletized.In a particular embodiment, the pelletized, oxygen-treated resin is usedto make a polyethylene film, the film having improved bubble stabilityand gauge uniformity.

In another aspect, the invention provides a process for producing apelletized polyethylene film resin having a bimodal molecular weightdistribution, the process including contacting ethylene underpolymerization conditions with a supported bimetallic catalyst toproduce a granular polyethylene resin having a bimodal molecular weightdistribution; conveying the resin through an extruder having a feed zonein which the resin is not melted, a melt-mixing zone in which at least aportion of the resin is melted, and a melt zone in which the resin is ina molten state, each zone being partially filled with the resin;contacting the molten resin in the melt zone with a gas mixture of 8 to40% by volume O₂; and pelletizing the oxygen-treated resin to form thepelletized polyethylene film resin

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a Kobe mixer.

FIG. 2 is a schematic diagram of a Farrel mixer.

DETAILED DESCRIPTION

The polyethylene resin preferably is a medium density polyethylene(MDPE), i.e., a polyethylene having a density typically in the range of0.930 g/cm³ to 0.945 g/cm³; or a high density polyethylene (HDPEs),i.e., a polyethylene having a density greater than 0.945 g/cm³ and up to0.970 g/cm³. The polyethylene can be a homopolymer or a copolymer, withpolymers having more than two types of monomers, such as terpolymers,also included within the term “copolymer” as used herein. Suitablecomonomers include α-olefins, such as C₃-C₂₀ α-olefins or C₃-C₁₂α-olefins. The α-olefin comonomer can be linear or branched, and two ormore comonomers can be used, if desired. Examples of suitable comonomersinclude linear C₃-C₁₂ α-olefins, and α-olefins having one or more C₁-C₃alkyl branches, or an aryl group. Specific examples include propylene;3-methyl-1-butene; 3,3-dimethyl-1-butene; 1-pentene; 1-pentene with oneor more methyl, ethyl or propyl substituents; 1-hexene with one or moremethyl, ethyl or propyl substituents; 1-heptene with one or more methyl,ethyl or propyl substituents; 1-octene with one or more methyl, ethyl orpropyl substituents; 1-nonene with one or more methyl, ethyl or propylsubstituents; ethyl, methyl or dimethyl-substituted 1-decene;1-dodecene; and styrene. It should be appreciated that the list ofcomonomers above is merely exemplary, and is not intended to belimiting. Preferred comonomers include propylene, 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene, 1-octene and styrene.

In a particular embodiment, the polyethylene resin has a bimodalmolecular weight distribution and/or a bimodal composition distribution.The resin can be produced in conventional processes, such as single ortandem gas phase fluidized bed reactors, or single or tandem slurry loopor supercritical loop reactors, using any catalyst capable of producingbimodal resins. The catalyst used is not particularly limited, and caninclude, for example, one or more Ziegler-Natta catalysts and/ormetallocene catalysts. Mixtures of catalysts can also be used. Inparticular, polymerization can be carried out with two or more differentcatalysts present and actively polymerizing at the same time, in asingle reactor. The two or more catalysts can be of different catalysttypes, such as a non-metallocene catalyst and a metallocene catalyst, toproduce a product resin having desirable properties. The catalysts canbe fed to the reactor separately or as a physical mixture, or eachcatalyst particle can contain more than one catalyst compound. When thecatalysts include two catalysts producing polymers of differentmolecular weight and/or different comonomer content, the polymer productcan have a bimodal distribution of molecular weight, comonomer, or both.Such bimodal products can have physical properties that are differentfrom those that can be obtained from either catalyst alone, or frompost-reactor mixing of the individual unimodal resins obtained from eachcatalyst alone.

For example, U.S. Pat. No. 5,525,678 discloses a catalyst including azirconium metallocene that produces a relatively low molecular weight,high comonomer-content polymer, and a titanium non-metallocene thatproduces a relatively high molecular weight, low comonomer-contentpolymer. Typically, ethylene is the primary monomer, and small amountsof hexene or other alpha-olefins are added to lower the density of thepolyethylene. The zirconium catalyst incorporates most of the comonomerand hydrogen, so that, in a typical example, about 85% of the hexene and92% of the hydrogen are in the low molecular weight polymer. Water isadded to control the overall molecular weight by controlling theactivity of the zirconium catalyst.

Other examples of suitable catalysts include Zr/Ti catalysts disclosedin U.S. Pat. No. 4,554,265; mixed chromium catalysts disclosed in U.S.Pat. Nos. 5,155,079 and 5,198,399; ZrNV and TiNV catalysts disclosed inU.S. Pat. Nos.5,395,540 and 5,405,817; the hafnium/bulky ligandmetallocene mixed catalysts disclosed in U.S. Pat. No. 6,271,323; andthe mixed metallocene catalysts disclosed in U.S. Pat. No. 6,207,606.

Typically, preferred bimodal resins include a narrow molecular weightdistribution low molecular weight (LMW) component produced by themetallocene catalyst and having a melt index I_(21.6) of 100 to 1000dg/min, and a high molecular weight (HMW) component produced by thenon-metallocene catalyst and having a flow index I_(21.6) of 0.1 to 1dg/min. The relative weight fraction of the HMW and LMW components canbe from about 1:9 to about 9:1. A typical resin has a HMW weightfraction of about 60% and a flow index of about 6.

The bimodal resin is processed in a mixer, such as a co- orcounter-rotating, intermeshing or non-intermeshing twin screw mixer.Such mixers are well-known in the art, and are commercially availablefrom various sources, such as Kobe and Farrel. The resin is fed to thefeeding zone of the mixer, where the temperature is below the meltingtemperature of the resin as the resin is compressed and conveyed towardthe melt-mixing zone. Typically, the temperature in the feeding zone is20 to 100° C., and is maintained by cooling the extruder walls. In themelt-mixing zone, the temperature is increased to at least partiallymelt the resin. In the melt zone, the temperature is sufficient to meltessentially all of the resin, to provide a molten polyethylene resin.Each zone is only partially filled with the resin; i.e., there are nocompletely filled zones. Although the terms “mixer” and “extruder” areoften used loosely and interchangeably, one skilled in the art willappreciate that mixers, such as the commercially available Kobe orFarrel mixers, operate at relatively low pressures, typically about 100psi or less, and the zones within the mixer are generally not completelyfilled with resin. In contrast, extruders, such as are commerciallyavailable from, for example, Werner-Pfleiderer, operate at much higherpressures, typically at least several hundred or several thousand psi,and the various zones within the extruder are generally completelyfilled with resin.

Although not limited to any particular mixer, a process of the inventionis illustrated now by specific reference to FIG. 1, showing a schematicdiagram of a Kobe mixer 10. Mixer 10 includes a feed zone 12, a mixingzone 14, and a melt-conveying zone 16. Resin and optional additives areprovided to mixer 10 in the feed zone 12, and the resin is conveyed in adownstream direction through the mixing zone 14 and the melt-conveyingzone 16. Gate 20 separates the mixing zone 14 from the melt-conveyingzone 16. An optional vent 22 is shown in FIG. 1 in the melt-conveyingzone 16. As described above, the resin is generally at least partiallymelted in mixing zone 14, and generally, but not necessarily,essentially completely melted in melt-conveying zone 16. The resin isconveyed through the mixer discharge 18 and further processed, such asby pelletizing.

Turning now to FIG. 2, specific reference is made to a Farrel mixer 30.Mixer 30 includes a feed zone 32, a mixing zone 34, and a melt-conveyingzone 36. Resin and optional additives are provided to mixer 30 in thefeed zone 32, and the resin is conveyed in a downstream directionthrough the mixing zone 34 and the melt-conveying zone 36. As describedabove, the resin is generally at least partially melted in mixing zone34, and generally, but not necessarily, essentially completely melted inmelt-conveying zone 36. The resin is conveyed through the mixerdischarge 38 and further processed, such as by pelletizing. The Farrelmixer does not have a gate such as gate 20 of the Kobe mixer separatingthe mixing zone from the melt-conveying zone. However, mixing zone 34and melt-conveying zone 36 are effectively separated by a narrowclearance region shown by dashed line 40 corresponding to the apex 42 ofmixing element 44. An optional dam (not shown) can be inserted betweenmixing zone 34 and melt-conveying zone 36 at the position of line 40.

The resin can be processed at a melt temperature of from a lower limitof 220° F. (104° C.) or 240° F. (116° C.) or 260° F. (127° C.) or 280°F. (138° C.) or 300° F. (149° C.) to an upper limit of less than 430° F.(221° C.) or less than 420° F. (216° C.) or less than 410° F. (210° C.)or less than 400° F. (204° C.), where the melt temperature is thetemperature at the downstream end of the mixing zone. For example, inFIG. 1, the melt temperature is the temperature at gate 20, and in FIG.2, the melt temperature is the temperature at the apex 42.

It should be appreciated that mixers other than the Kobe and Farrelmixers illustrated herein can be used.

The resin is contacted with oxygen in the melt-conveying zone. Theoxygen can be provided, for example, through one or more gas inletports. Referring to FIG. 1, for example, in some embodiments, oxygen canbe provided through one or more inlets 24. Referring to FIG. 2, forexample, in some embodiments, oxygen can be provided through one or moreinlets 46. It should be appreciated that these specific inlet positionsare merely exemplary.

Oxygen can be provided as a continuous flow of gas or, alternatively,oxygen can be provided intermittently.

Oxygen gas can be provided as an essentially pure gas, or as part of agas mixture, such as air. The oxygen can be provided in a pre-mixed gasmixture, or co-fed to the extruder with a diluent gas, adjusting theamount of oxygen in the resulting mixture by adjusting relativeoxygen/diluent gas flow rates. For example, oxygen and nitrogen can befed to the extruder at separately metered flow rates to provide oxygento the extruder at the desired concentration.

After the oxygen treatment, or “tailoring”, the resin can be extrudedthrough a die and pelletized and cooled, or can be directly extrudedwithout pelletization to form a film, such as by a cast or blown filmprocess.

Various additives can also be introduced into the extruder, as isconventional in the art.

EXAMPLES

Film gauge was measured according to ASTM D374-94 Method C.

Film gauge variation was determined using a Measuretech Series 200instrument. This instrument measures film thickness using a capacitancegauge. For each film sample, ten film thickness datapoints are measuredper inch of film as the film is passed through the gauge in a transversedirection. Three film samples were used to determine the gaugevariation. The gauge variation was determined by dividing the full rangeof film thickness (maximum minus minimum) by the average thickness, anddividing the result by two. The gauge variation is presented as apercentage change around the average.

Dart Drop Impact values were measured using the procedures in ASTMD1709-98 Method A, except that the film gauge was measured according toASTM D374-94 Method C.

Elmendorf Tear strength (machine direction, “MD”, and transversedirection, “TD”) were measured using the procedures in ASTM D1922-94a,except that the film gauge was measured according to ASTM D374-94 MethodC.

The term “Melt Index” refers to the melt flow rate of the resin measuredaccording to ASTM D-1238, condition E (190° C., 2.16 kg load), and isconventionally designated as I_(2.16). The term “Flow Index” refers tothe melt flow rate of the resin measured according to ASTM D-1238,condition F (190° C., 21.6 kg load), and is conventionally designated asI_(21.6). Melt index and flow index have units of g/10 min, orequivalently dg/min. The term “MFR” refers to the ratioI_(21.6)/I_(2.16), and is dimensionless.

Specific Energy Input (SEI) refers to the energy input to the main driveof the extruder, per unit weight of melt processed resin, and isexpressed in units of hp·hr/lb or kW·hr/kg.

“Elasticity” as used herein is the ratio of G′ to G″ at a frequency of0.1 s⁻¹, where G′ and G″ are the storage (or elastic) and loss (orviscous) moduli, respectively. G′ and G″ were measured according to ASTMD-4440-84. Measurements were made at 200° C. using a Rheometrics RMS 800oscillatory rheometer.

Density (g/cm³) was determined using chips cut from plaques compressionmolded in accordance with ASTM D-1928-96 Procedure C, aged in accordancewith ASTM D618 Procedure A, and measured according to ASTM D1505-96.

In some examples, “Bubble Stability” was determined visually andqualitatively, and is designated as poor, good, etc. In examples wherein“Bubble Stability” is given a numerical value, the bubble stability wasdetermined as the maximum linespeed obtainable before the onset ofinstability was observed, as evidenced by vertical or horizontaloscillations at a blowup ratio (BUR) of 4:1.

Oxygen was provided in an oxygen-nitrogen gas mixture. The oxygen levelwas controlled by varying the relative flows of oxygen and nitrogen. Theoxygen level reported in the data tables was calculated from thevolumetric flow rates of air and nitrogen.

The data tables do not include film properties for the base (untailored)resin, since it is usually not possible to fabricate film (i.e., developa stable bubble) out of the untailored resin. As such, no attempt wasmade to compound granular resin under non-tailored conditions and run iton the Alpine film line used in the following examples.

Examples 1-4

A medium density polyethylene (MDPE) bimodal resin was produced using abimetallic catalyst in a single gas phase fluidized-bed reactor. Thebimetallic catalyst was a Ziegler-Natta/Metallocene catalyst asdescribed in U.S. Pat. No. 6,403,181. The resin had a density of 0.938g/cm³, a melt index I_(2.16) of 0.07 dg/min, a flow index I_(21.6) of6.42 dg/min, and an MFR (I_(21.6)/I_(2.16)) of 92. Oxygen tailoring ofthe bimodal resin was carried out on a 4 inch (10 cm) diameter Farrel4LMSD compounder. The Farrel 4LMSD compounder has a 5 L/D rotor.Referring now to FIG. 2, an oxygen/nitrogen gas mixture (21% O₂ byvolume) was added at a flow rate of 10 standard ft³/hr (0.3 m³/hr) inthe melt-conveying zone 36 at 0.5 L/D from the end of the rotor.Optionally, a flow dam can be inserted at about 1.0 L/D 40 from themachine discharge end, and oxygen is injected after the flow dam in themelt conveying zone 36. Several samples of resin were thus processed atdifferent melt temperatures.

Monolayer blown films were produced from the tailored resins on a 50 mmAlpine film line with a 100 mm die and 1 mm die gap, at a rate of 120lb/hr (54 kg/hr), a blow up ratio (BUR) of 4:1, and a 28 inch frostheight. These Examples are summarized in Table 1. TABLE 1 Base ExampleNo. Resin 1 2 3 4 Compounding Conditions SEI,^((a)) actual (hp · hr/lb,0.112, 0.119, 0.125, 0.137, kW · hr/kg) 0.184 0.196 0.206 0.225 O₂ level(volume %) 21 21 21 21 Melt T (° C.) 239 241 243 268 ResinCharacteristics Flow Index, I_(21.6) (dg/min) 6.42 5.54 5.57 5.61 5.14Melt Index, I_(2.16) (dg/min) 0.07 0.061 0.055 0.056 0.046 MFR,I_(21.6)/I_(2.16) 92 91 101 100 112 Rheology Elasticity^((b)) at 0.1 s⁻¹0.51 0.61 0.64 0.66 0.74 Increase in Elasticity (%) 20 26 31 47Processability Melt Pressure 7590, 7760, 7760, 8020, (psi, MPa) 52.353.5 53.5 55.3 Bubble Stability at 0.5 mil poor some good good [13 μm]vert.^((c)) Film Properties 1 mil (25 μm) Gauge Dart Drop Impact (g,g/mil, 473, 482, 527, 479, g/μm) 473, 482, 527, 479, 18.6 19.0 20.7 18.9Elmendorf Tear, 33, 32, 32, 32, MD (g/mil, g/μm) 1.3 1.3 1.3 1.3Elmendorf Tear, 338, 304, 258, 211, TD (g/mil, g/μm) 13.3 12.4 10.2 8.310.5 mil (13 μm) Gauge Dart Drop Impact (g, g/mil, 449, 548, 518, 401,g/μm) 898, 1096, 1036, 802, 35.4 43.1 40.8 31.6 Elmendorf Tear, 18, 25,23, 28, MD (g/mil, g/μm) 0.71 0.98 0.91 1.1 Elmendorf Tear, 203, 128,130, 110, TD (g/mil, g/μm) 7.99 5.04 5.12 4.33Specific Energy Input;^((b))G′/G″;^((c))some vertical oscillations observed

As the degree of oxygen tailoring increases, as measured by an increasein specific energy input (SEI), the bubble stability of the resinimproves. The change in resin characteristics as a result of oxygentailoring is reflected in the increase in elasticity of up to 47percent. It is surprising that with the use of a very high level ofoxygen (21 volume %), an excellent balance of bubble stability and filmproperties is achieved.

Examples 5-9

A high density polyethylene (HDPE) bimodal resin was produced using abimetallic catalyst in a single gas phase fluidized-bed reactor. Thebimetallic catalyst was a Ziegler-Natta/Metallocene catalyst asdescribed in U.S. Pat. No. 6,403,181. The resin had a density of 0.946g/cm³, a melt index I_(2.16) of 0.066 dg/min, a flow index I_(21.6) of5.81 dg/min, and an MFR (I_(21.6)/I_(2.16)) of 88. Oxygen tailoring ofthe bimodal resin was carried out as described above. Several samples ofresin were thus processed at different melt temperatures. Monolayer castfilms were produced from the tailored resins as described above. TheseExamples are summarized in Table 2. TABLE 2 Base Example No. Resin 5 6 78 9 Compounding Conditions Flow Dam at 4th Segment of no no No yes yesMixer SEI,^((a)) actual (hp · hr/lb, 0.115, 0.129, 0.147, 0.131, 0.145,kW · hr/kg) 0.189 0.212 0.242 0.215 0.238 O₂ level (volume %) 21 21 2121 21 Melt T (° C.) 231 253 281 256 282 Resin Characteristics FlowIndex, I_(21.6) (dg/min) 5.81 5.69 5.46 5.33 5.52 5.38 Melt Index,I_(2.16) (dg/min) 0.066 0.062 0.051 0.042 0.054 0.043 MFR,I_(21.6)/I_(2.16) 88 92 108 127 103 125 Rheology Elasticity^((b)) at 0.1s⁻¹ 0.52 0.54 0.69 0.76 0.67 0.79 Increase in Elasticity (%) 4 33 46 2951 Processability Melt Pressure 6500, 6450, 6400, 6300, 6250, (psi, MPa)44.8 44.5 44.1 43.4 43.1 Bubble Stability^((c)) <200,280, >300, >300, >300, (ft/min, m/s) <1.0 1.4 >1.5 >1.5 >1.5 FilmProperties Dart Drop Impact, 1 mil 270, 350, 345, 300, 315, [25 μm] 270,350, 345, 300, 315, (g, g/mil, g/μm) 10.6 13.8 13.6 11.8 12.4 Dart DropImpact, 0.5 mil [13 μm] 410, 420, 360, 390, 270, 820, 840, 720, 780,540, (g, g/mil, g/μm) 32.3 33.1 28.3 30.7 21.3Specific Energy Input^((b))G′/G″maximum linespeed achieved before onset of instability

As the severity of tailoring, as measured by SEI or by melt temperature,increased, the bubble stability as measured by maximum linespeedincreased. The presence of a flow dam also increased tailoring. It wassurprisingly found that even with use of a high level of oxygen (21volume %), a balance of bubble stability (maximum linespeed greater than300 ft/min (1.5 m/s)) and properties was achieved.

Examples 10-12C

A high density polyethylene (HDPE) bimodal resin was produced using abimetallic catalyst in a single gas phase fluidized-bed reactor. Thebimetallic catalyst was a Ziegler-Natta/Metallocene catalyst asdescribed in U.S. Pat. No. 6,403,181. Oxygen tailoring of the bimodalresin was carried out as described above.

ExxonMobil HD-7755 was used as a comparative resin (Example 12C).ExxonMobil HD-7755 is a bimodal ethylene copolymer produced in a seriestandem reactor. ExxonMobil HD-7755 has a density of 0.952 g/cm³, a meltindex I_(2.16) of 0.055 dg/min, and a flow index I_(21.6) of 9 dg/min.ExxonMobil HD-7755 is available from ExxonMobil Chemical Company,Houston, Tex. This example is summarized in Table 3.

Monolayer blown films were produced from the tailored resins on a 50 mmAlpine film line with a 120 mm monolayer die and nominal 1.2 mm die gapat a rate of 200 lb/hr (90.7 kg/hr), a blow up ratio (BUR) of 4:1, and a48-52 inch (122-132 cm) frost height. These Examples are summarized inTable 3. TABLE 3 Base Example No. Resin^((a)) 10 11 12C CompoundingConditions SEI,^((b)) actual (kW · hr/kg) 0.176 0.181 O₂ level (volume%) 9.1 9.2 Mixer Metal T (° C.) 266 277 Rate (10³ kg/hr) 25 25 BarrelPulse Water Cooling Off On Resin Characteristics Flow Index, I_(21.6)(dg/min) 6.56 6.14 6.44 14.1 Melt Index, I_(2.16) (dg/min) 0.083 MFR,I_(21.6)/I_(2.16) 84 106 124 186 Density (g/cm³) 0.9519 0.9512 0.951Rheology Elasticity^((c)) at 0.1 s⁻¹ 0.498 0.612 0.740 Increase inElasticity (%) — 23 49 Processability Melt Pressure 7775, 7610, 7175,(psi, MPa) 53.61 52.47 49.47 Film Properties Gauge Variation (%) 16 1120 Dart Drop Impact (g, g/mil,  204,  169,  203, g/μm)  408,  338,  406,16.1 13.3 16.0 Elmendorf Tear,  11,  11,  11, MD (g/mil, g/μm) 0.43 0.430.43Values represent the average of two samplesSpecific Energy Input^((c))G′/G″

All patents, test procedures, and other documents cited herein,including priority documents, are fully incorporated by reference to theextent such disclosure is not inconsistent with this invention and forall jurisdictions in which such incorporation is permitted.

1. A process for extruding a bimodal polyethylene resin, the processcomprising: (a) providing a polyethylene homopolymer or copolymer resinhaving a bimodal molecular weight distribution; (b) conveying the resinthrough an extruder having a feed zone in which the resin is not melted,a melt-mixing zone in which at least a portion of the resin is melted,and a melt zone in which the resin is in a molten state, each zone beingpartially filled with the resin; and (c) contacting the molten resin inthe melt zone with a gas mixture comprising 8 to 40% by volume O₂. 2.The process of claim 1, wherein the bimodal polyethylene resin is acopolymer of ethylene and at least one comonomer selected from the groupconsisting of C₃ to C₁₂ alpha-olefins.
 3. The process of claim 1,wherein the bimodal polyethylene resin has a density of at least 0.93g/cm³.
 4. The process of claim 1, wherein the bimodal polyethylene resinhas a density of at least 0.945 g/cm³.
 5. The process of claim 1,wherein the bimodal polyethylene resin has a density of from 0.93 to0.97 g/cm³.
 6. The process of claim 1, wherein the gas mixture comprises10 to 35% by volume O₂.
 7. The process of claim 1, wherein the gasmixture comprises 15 to 25% by volume O₂.
 8. The process of claim 1,wherein the step of providing comprises contacting monomers comprisingethylene under polymerization conditions with a supported bimetalliccatalyst to produce a polyethylene resin having a bimodal molecularweight distribution.
 9. The process of claim 8, wherein the supportedbimetallic catalyst comprises a non-metallocene transition metalcatalyst and a metallocene transition metal catalyst.
 10. The process ofclaim 8, wherein the monomers comprise ethylene and at least one C₃-C₁₂alpha olefin.
 11. The process of claim 1, further comprising after step(c) pelletizing the oxygen-treated resin.
 12. A process for producing apelletized polyethylene film resin having a bimodal molecular weightdistribution, the process comprising: (a) contacting ethylene underpolymerization conditions with a supported bimetallic catalyst toproduce a polyethylene resin having a bimodal molecular weightdistribution; (b) conveying the resin through an extruder having a feedzone in which the resin is not melted, a melt-mixing zone in which atleast a portion of the resin is melted, and a melt zone in which theresin is in a molten state, each zone being partially filled with theresin; (c) contacting the molten resin in the melt zone with a gasmixture comprising 8 to 40% by volume O₂; and (d) pelletizing theoxygen-treated resin to form the pelletized polyethylene film resin. 13.The process of claim 12, wherein the bimodal polyethylene resin is acopolymer of ethylene and at least one comonomer selected from the groupconsisting of C₃ to C₁₂ alpha-olefins.
 14. The process of claim 12,wherein the bimodal polyethylene resin has a density of at least 0.93g/cm³.
 15. The process of claim 12, wherein the bimodal polyethyleneresin has a density of at least 0.945 g/cm³.
 16. The process of claim12, wherein the bimodal polyethylene resin has a density of from 0.93 to0.97 g/cm³.
 17. The process of claim 12, wherein the gas mixturecomprises 10 to 35% by volume O₂.
 18. The process of claim 12, whereinthe gas mixture comprises 15 to 25% by volume O₂.
 19. The process ofclaim 12, wherein the supported bimetallic catalyst comprises anon-metallocene transition metal catalyst and a metallocene transitionmetal catalyst.